Blood vessel occlusions are commonly treated by mechanical means using implantable medical devices such as stents. Stents can be used not only as a mechanical intervention but also as a vehicle for providing therapeutic agents.
There are several characteristics that are important for implantable medical devices, such as stents, including high radial strength, good fracture toughness, and fast degradation. Some polymers that may be suitable for use in implantable medical devices have potential shortcomings with respect to some of these characteristics, in particular, fracture toughness and degradation rate. Some crystalline or semi-crystalline polymers that are glassy or have a glass transition temperature (Tg) above body temperature are particularly attractive as stent materials due to their strength and stiffness at physiological conditions. Such glassy polymers can be absorbed through chemical degradation, such as hydrolysis. Physiological conditions refer to conditions that an implant is exposed to within a human body. Physiological conditions include, but are limited to, human body temperature, approximately 37° C. The toughness of such polymers can be lower than desired, in particular, for use in stent applications. For example, polymers such as poly(L-lactide) (PLLA) are stiff and strong, but tend to be brittle under physiological conditions. These polymers can exhibit a brittle fracture mechanism at physiological conditions in which there is little or no plastic deformation prior to failure. As a result, a stent fabricated from such polymers can have insufficient toughness for the range of use of a stent.
Furthermore, some biodegradable polymers have a degradation rate that is slower than desired for certain stent treatments. As a result, the degradation time of a stent made from such polymers can be longer than desired. For example, a stent made from a semicrystalline polymer such as PLLA can have a degradation time between about two and three years. In some treatment situations, a shorter degradation time is desirable, for example, less than 6 months or a year.
Other potential problems with polymeric stents include creep, stress relaxation, and physical aging, which result from relaxation or rearrangement of polymer chains. Creep refers to the gradual deformation that occurs in a polymeric construct subjected to an applied load. Creep, for example, can result in an expanded stent can retracting radially inward, reducing the effectiveness of a stent in maintaining desired vascular patency.
There is, therefore, an on-going need for implantable medical devices made of polymers that meet all criteria for such implements with regard to radial strength, fracture toughness, biodegradation rate, etc. The current invention provides such implantable medical devices